1. Field of the Invention
The present invention relates to fuel pellets for nuclear reactors, and particularly fuel pellets configured to minimize pellet chipping and cracking and to protect the integrity of fuel rods in which they are housed.
2. Related Art
Conventional nuclear fuel pellets are fabricated by compressing suitable powders into a generally cylindrical mold. The compressed material is then sintered, resulting in a substantial reduction in volume. The resulting pellet is generally cylindrical and often has concave surfaces at each end as a result of the compression.
The cylindrical fuel pellets are easily chipped, particularly at the rim, when fuel rods in which they are housed are subjected to normal flexing during handling or vibrational flexing resulting from coolant flow. The dislodged fuel pellet chips produce localized hot spots in the fuel rod which are often responsible for local fuel rod cladding failure. When the fuel rod cladding fails, radioactive gases and materials contained in the released fission gas are released into both the coolant and the atmosphere.
U.S. Pat. No. 4,120,752 to Ocken discloses a mixed oxide fuel pellet. It has been found that large agglomerates of PuO.sub.2 in the vicinity of the outer surface of the fuel pellet result in local hot spots in the cladding. In order to prevent localized melting and perforation of the cladding due to the PuO.sub.2 agglomerates, a fuel pellet is fabricated from a generally cylindrical pellet of PuO.sub.2 surrounded by an annular UO.sub.2 casing. The outer UO.sub.2 casing is free of PuO.sub.2 so as to avoid localized melting and perforation of the fuel rod cladding due to the PuO.sub.2 agglomerates. This structure does not solve the problem of fuel pellet chipping and cracking occasioned as a result of forces acting on the pellet during vibrational flexing. The sharp edges of the generally cylindrical shape of the pellet are known to be subjected to various types of loading conditions which are particularly vulnerable to chipping and cracking. The UO.sub.2 annular casing does not mitigate this problem.
U.S. Pat. No. 3,415,911 to Lloyd discloses generally cylindrical pellets inserted into a tubular fuel rod for use in a nuclear reactor. In order to achieve intimate contact between the fuel pellet and the fuel rod cladding, the stacked fuel rods are subjected to ultrasonic vibration or thermal shock in order to crack the fuel pellets contained in the fuel rod. By cracking the fuel pellets, the cross-sectional dimensions of the fuel pellet bodies are increased relative to the fuel rod cladding, thereby achieving more intimate contact between the fuel pellets and the fuel rod.
U.S. Pat. No. 3,192,621 to Bauer et al discloses cylindrical fuel pellets having arcuate "rebates" which form annular channels when the pellets are stacked on top of one another. The annular channel provides an area into which the fuel cladding may expand. The arcuate rebates formed in the cylindrical pellets, however, increase the number of sharp edges on each individual pellet. Therefore, there is a greater possibility of fuel pellet chipping and cracking.